<!DOCTYPE html>
<html>
 <head>
  <meta charset="utf-8" /> 
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title></title>
 </head>
 <body>
  <div>
   <div>
    
    <!--<input type="button" id="open" value="open"></input>
    <input type="button" id= "save"  value = "save"></input> <input type="button" id= "color" value="color"></input> -->
    <input type="button"  value="size"></input>
    <input type="text" id="size"  onchange="sizeChange()"></input>
    <input type="button" id="clear" value="clear"></input>
    <input type="button" id="eraser" value="eraser" onclick="doEraser()"></input>
    <select id = "shape" onchange="shapeChange()">
     <option value = "99">shape</option>
     <option value = "1">rectangle</option>
     <option value = "0">circle</option>
     <option value = "2">line</option>
    </select>

    
    <input  id="color" type="color"/>
   </div>
   <canvas id="myCanvas" style=" border:1px solid;" width="800" height="500"></canvas>
   </div>
 </body>
 <script language="JavaScript">
  var shap = 99; //0 is circle; 1 is rectangle
  var orignalX, orignalY;//the coordinate of mouse down
  var lastX, lastY;//the coordinate of last mouse position
  var isMouseDown = false; // flag of mouse pressing down
  var myCanvas = document.getElementById("myCanvas");
  var context = myCanvas.getContext('2d');
  var width = myCanvas.width, height = myCanvas.height;
  var data;//storing last canvas' content
  context.strokeStyle = "black";
  context.strokeWidth=1;
  context.lineWidth = 1;

  
  document.getElementById('color').onchange = function(){
           context.strokeStyle = this.value
  };
  
  function doEraser(){
  context.strokeStyle = "white";
  shap = 2;
  }
  function sizeChange(){
    
    context.lineWidth = parseInt(document.getElementById('size').value);
    
  }
  
  function shapeChange(){
  context.strokeStyle = "black";
        var myselect = document.getElementById("shape");
  var index=myselect.selectedIndex ;  
  var myvalue  = myselect.options[index].value;
  var mytext=myselect.options[index].text; 
  shap = parseInt(myvalue);
  
       }

 

  function myCanvasMouseDown(event) {
   //event.preventDefault();
    
   if(event.button == 0) {
    orignalX = event.offsetX;
    orignalY = event.offsetY;
    context.moveTo(orignalX,orignalY); 
    data = context.getImageData(0, 0, width, height);
    isMouseDown = true;
    
   }
  }

  function myCanvasMouseMove(event) {
   if (isMouseDown){
    //event.preventDefault();
    
    switch(shap){
     case 0:
         context.clearRect(0,0,width,height);
      context.putImageData(data,0,0);
      lastX = event.offsetX;
      lastY = event.offsetY;
      context.beginPath();
      context.arc(orignalX+(lastX-orignalX)/2,orignalY+(lastY-orignalY)/2,Math.abs(lastX-orignalX)/2,0,Math.PI * 2,true);
      context.stroke();
      context.closePath();
      break;
     case 1:
      context.clearRect(0,0,width,height);
      context.putImageData(data,0,0);
      lastX = event.offsetX;
      lastY = event.offsetY;
      context.strokeRect(orignalX, orignalY, lastX-orignalX, lastY-orignalY);
      break;
     case 2:
         lastX = event.offsetX;
         lastY = event.offsetY;
           context.lineTo(lastX, lastY); //根据鼠标路径绘画  
            context.stroke();   //立即渲染  
         break;
     
    }
   }
  }

  function myCanvasMouseUp(event) {
   if (isMouseDown){
    //event.preventDefault();
    
    context.clearRect(0,0,width,height);
    context.putImageData(data,0,0);
    lastX = event.offsetX;
    lastY = event.offsetY;
    switch(shap){
     case 0:
      context.beginPath();
      context.arc(orignalX+(lastX-orignalX)/2,orignalY+(lastY-orignalY)/2,Math.abs(lastX-orignalX)/2,0,Math.PI * 2,true);
      context.stroke();
      context.closePath();
      break;
     case 1:
      context.beginPath();
      context.strokeRect(orignalX, orignalY, lastX-orignalX, lastY-orignalY);
      context.closePath();
      break;
     case 2:
      
        context.lineTo(lastX, lastY); //根据鼠标路径绘画  
         context.stroke();   //立即渲染 
      
      break;
    }
    isMouseDown = false;
    lastX = null;
    lastY = null;
    orignalX = null;
    orignalY = null;
    data = context.getImageData(0, 0, width, height);
    context.beginPath();
    context.clearRect(0,0,width,height);
    context.putImageData(data,0,0);
    context.closePath();
   }
  } 
  myCanvas.addEventListener("mousedown", myCanvasMouseDown, false);
  myCanvas.addEventListener("mousemove", myCanvasMouseMove, false);
  myCanvas.addEventListener("mouseup", myCanvasMouseUp, false);
 </script>
 
</html>